For many types of interactive applications, touchscreens are ideal interface devices. This is because they are intuitive, reprogrammable, and capable of being environmentally sealed. Also, touchscreens occupy a relatively small amount of space. Thus, it would be desirable to incorporate touchscreen functionality in applications utilizing liquid crystal display (LCD) panels.
The configuration of a typical LCD device is illustrated in FIGS. 1A and 1B. As shown in FIG. 1A, a typical LCD device 1 includes a liquid crystal (LC) layer 20 sandwiched between two polarizing filters 30A and 30B (hereafter “polarizers”). The LC layer is protected by a transparent front protective sheet 10, e.g., a glass plate. For a backlit LCD device 1, behind the LC and polarizing layers are a light diffusing film 40 (hereafter “diffuser”), a backlight source 50, and a reflective surface 60. However, in a reflective-type LCD device 1, the diffuser 40 and backlight source 50 would be omitted (thus, these layers are illustrated by dotted lines in FIG. 1A). A casing or enclosure 70 is provided to hold the aforementioned layers in place. FIG. 1B illustrates an exploded view of the stack of LCD layers described above. The specification may collectively refer to these layers as the “LCD stack” of a backlit LCD device (including diffuser 40 and backlight source 50) or a reflective-type LCD device (without diffuser 40 or backlight source 50).
In a typical backlit LCD device (also referred to as a “transmissive” LCD device), the backlight is emitted directly from source 50 and reflected from reflective surface 60 to the diffuser 40. The diffuser 40 diffuses this light to make the intensity or brightness more uniform across the LCD. Polarizers 30A and 30B are cross-polarized with respect to each other.
Thus, the backlight polarized by polarizer 30B must be rotated to some extent by LC layer 20 in order to pass through polarizer 30A. The degree to which the LC layer 20 rotates the light is dependent upon the amount of voltage applied across the various liquid crystal molecules in the LC layer 20. For instance, a pair of electrodes (not shown) may be positioned across each LC cell to apply an appropriate voltage to “twist” the corresponding LC molecules, thereby rotating the backlight to pass through. In backlit LCD devices, numbers and characters are displayed according to the LC cells that allow light to pass through polarizers 30A and 30B.
FIGS. 2A and 2B illustrate the different types of backlight sources 50 that can be implemented in a typical backlit LCD device 1. Specifically, FIG. 2A illustrates a side view of a backlit LCD device 1, while FIG. 2B shows a cross-sectional view at CV.
As illustrated in FIGS. 2A and 2B, the backlight source 50 may include a combination of “pinpoint” light sources 52 (e.g., LED lamps) and/or cold cathode fluorescent lamps (CCFLs) 56. Furthermore, different types of diffusers 40 may be used. For instance, these figures show an edge-lit light guide/diffuser 44 dedicated specifically to the pinpoint LED sources 52. Also, a light diffusing sheet 42 may be implemented in front of the CCFL sources 56.
As shown in FIGS. 2A and 2B, the pinpoint light sources 52 are configured to emit light into the edge-lit light guide/diffuser 44, which is situated parallel to the LC layer 20. As such, the edge-lit light guide/diffuser 44 is intended to distribute the light from the pinpoint light sources 52 more uniformly. The combination of the edge-lit light guide/diffuser 44 and LED light sources 52 is generally referred to as an LED edge-lit light guide assembly.
However, as mentioned above, an alternative to backlit LCD devices are reflective-type LCDs. In a reflective-type LCD device, the LC layer 20 is illuminated by external light. Referring again to FIGS. 1A and 1B, after passing through the LC layer 20 and polarizers 30A and 30B, the external light is reflected (and optionally diffused) by the reflective surface 60 back to the viewer. In such devices, the cells in the LC layer 20 are configured, by default, to allow light to pass through. Thus, numbers and characters are displayed using LC cells, which are charged by electrodes (not shown) to block light from passing through polarizers 30A and 30B.
Generally, previous attempts to utilize a touchscreen in conjunction with an LCD panel (backlit or reflective-type) require additional panels or layers to be added to the front protective sheet of the LCD device. This is disadvantageous because it reduces the amount of output light from the LCD, increases the complexity of the resultant device, and reduces overall system reliability.